The present invention relates to a corrosion preventing device for in-pile graphite structures, fuel and the like of a helium gas-cooled nuclear reactor against water or air ingress accidents.
In a helium gas-cooled reactor, graphite is employed to form in-pile structures or fuel. Thus, because H.sub.2 O, and O.sub.2 and the like included in helium gas supplied into the reactor as a coolant have the action of accelerating oxidation corrosion of the graphite structures, a coolant purifying device formed of activated carbon or a molecular sieve was provided in a coolant supply pipeline system in order to remove oxidizing impurities such as H.sub.2 O and O.sub.2 from the helium gas supplied into the reactor. However, since this coolant purifying device was designed only to remove oxidizing impurities from the supplied coolant gas in a normal operational state, it is not able to remove O.sub.2 and H.sub.2 O when a large quantity of air or air-helium mixed gas rushes abruptly into the reactor, as may occur if, for instance, the primary cooling pipeline accidentally ruptures. Particularly, if an accident occurs where the pressure drops abruptly, gas may rush into the reactor without passing through the coolant purifying device. In such a case, there is possibility that graphite members of the reactor may suffer severe oxidation corrosion. These circumstances will be described with reference to FIG. 1.
FIG. 1 is a vertical sectional view showing schematically a helium gas-cooled reactor. In FIG. 1, reference numeral 11 designates a ruptured portion of a primary cooling gas supply pipeline. An inwardly rushing gas flowing through the ruptured portion 11 is designated by an arrow 12. Other arrows designate the flow of the coolant helium gas inside the reactor. The air or helium-air mixed gas rushing through ruptured portion 11 generally includes both O.sub.2 and H.sub.2 O components in relatively high concentrations. The oxidizing action of O.sub.2 and H.sub.2 O contained in the invading gas is accelerated by the elevated temperature inside the reactor. Accordingly, in-pile structures, such as a plenum post 13 and fuel 14, are oxidized and corroded. The temperature inside the reactor will be lowered by a cooling system panel 15 provided in the rear. However, it takes a relatively long time for the reactor to be cooled sufficiently because of the large thermal capacity of the in-pile structures. During the cooling period, oxidizing of the graphite members of the pile unavoidably will occur.